Metal, ceramic and cermet articles formed from low viscosity aqueous slurries

ABSTRACT

Methods for producing metal, ceramic and cermet articles from low viscosity suspensions and the articles produced thereby. The articles include micro diameter hollow fibers, tubes having hollow walls, solid and hollow sheets, and open cell foams. The articles are useful for filters, catalyst media, fuel cell electrodes, body implantation devices, structural materials, vibration and noise control, heat exchangers, heat sinks, heat pipes, heat shields and other applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for producing metal, ceramic andcermet articles from low viscosity suspensions and the articles producedthereby. The articles include micro diameter hollow fibers, tubes havinghollow walls, solid and hollow sheets, and open cell foams. The articlesare useful for filters, catalyst media, fuel cell electrodes, bodyimplantation devices, structural materials, vibration and noise control,heat exchangers, heat sinks, heat pipes, heat shields and otherapplications.

2. Description of the Related Art

U.S. Pat. No. 4,268,278 described a method of preparing inorganic hollowfibers by first forming a polymeric precursor hollow fiber laden withthe inorganic material, removing the polymer and sintering the inorganicmaterial. The hollow fiber had a radially anisotropic void volume wallstructure.

U.S. Pat. Nos. 5,011,566 and 5,298,298 described a method of preparing amicroscopic tube by first depositing a oxidation resistant material onthe surface of an oxidizable fiber, preferably by chemical vapordeposition, said deposition occurring in an inert environment. Thecoated fibers were placed in an oxidizing environment and the base fiberwas removed by oxidation, the remaining coating forming a hollow tube.

U.S. Pat. No. 5,352,512 described a method of preparing a microscopictube by first depositing a solvation resistant material on the surfaceof an dissolvable fiber, preferably by chemical vapor deposition, saiddeposition occurring in an inert environment. The coated fibers wereplaced in a solvating environment and the base fiber was removed, theremaining coating forming a hollow tube.

U.S. Pat. No. 6,194,066 described microscopic tubes having porous wallsor multi-layer walls prepared by methods similar to those described inU.S. Pat. Nos. 5,011,566, 5,298,298 and 5,352,512.

U.S. Pat. No. 6,458,231 described methods of manufacturing microtubesthat had peripheral geometries that were not uniform along the tubeaxis.

Methods for the preparation of porous ceramic bodies through coating offoam substrates with slurries of ceramic particles have been describedfor example in U.S. Pat. Nos. 3,090,094, 3,097,930, 3,893,917,3,893,917, 3,947,363, 3,962,081, 3,993,495, 4,004,933, 4,024,212,4,056,586, 4,075,303, 4,113,241, 4,154,689, 4,158,684, 4,265,659,4,343,704, 4,610,832, 4,803,025, 4,833,106, 4,866,011, 4,885,263,4,923,830, 4,975,191, 5,039,340, 5,177,035, 5,185,297, 5,429,780,5,676,833, 5,705,118, 6,426,163, 6,932,925 and United States PatentApplication 20040077480.

These prior art methods were limited in the combination of pore densityand body thickness that could be achieved. U.S. Pat. Nos. 4,056,586 and3,893,917 reported that the maximum body thickness that could beachieved at a pore density of 100 pores/in (39.37 pores/cm) was 10 cm.In U.S. Pat. No. 4,075,303 using a slurry viscosity of 1,000 to 80,000centipoises (1 to 80 Pa-s), the maximum pore density that could beachieved at a thickness of 10 cm was 25-35 pores/in (9.84-13.78pores/cm).

Methods for the preparation of porous metal or inorganic bodies throughcoating of foam substrates with slurries of metal particles or inorganicparticles have similarly been described for example in U.S. Pat. Nos.3,111,396, 3,408,180, 3,946,039, 4,560,621, 5,640,669, 5,881,353,5,951,791, 6,399,528, 6,387,149, 6,524,522, 6,706,239, 6,840,978 B2, andJapanese Kokai Patent Publication JP6271904. JP6271904 described use oflow viscosity slurries of metal particles in liquid phenolic resins.

The maximum body thickness that could be achieved with slurries of metalparticles was reported in U.S. Pat. No. 5,640,669 to be 0.25 cm at apore density of 50 pores/in (19.69 pores/cm).

By way of further background, the penetration of porous media by pureliquids is sensitive to the viscosity of the liquid as expressed by theLucas-Washburn equation. To wit,

$x^{2} = {\frac{\sigma\; r\;\cos\;\theta}{2\;\mu}t}$

where x is the distance penetrated by the liquid, σ is the surfacetension of the liquid, μ is its viscosity, θ is the contact anglebetween the liquid and the surface of the capillary, r is the radius ofthe capillary and t is the penetration time. (Ref.: Marmur et al.,“Characterization of Porous Media by the Kinetics of Liquid PenetrationThe Vertical Capillaries Model”, J. Colloid and Interface Sci., 199299-304 (1997)). This sensitivity of penetration distance to viscosityappears not to have been recognized in the prior art cited above forpreparing porous inorganic bodies by coating foam substrates.

Each of the prior art methods represented progress toward the goals towhich they were directed. However, none described the specific methodsor articles of this invention and none recognized the problem andsatisfied all of the needs met by this invention. Prior art methods ofproducing hollow metal fibers produced fibers with radially anisotropicwalls or they were limited in the length of the fibers that could beproduced with impervious walls of uniform radial composition. A needexists for hollow metal fibers having isotropic impervious walls ofuniform radial composition. Further, a need exists for continuous hollowmetal fibers of indefinite length.

Prior art methods of producing porous metal or ceramic articles usingslurries were limited in their ability to produce both high pore densityand thick sections simultaneously. Some prior art methods used solventsthat adversely affected the environment and/or left carbon residue inthe final article. Prior art processes seem not to have producedcontinuous articles of indefinite length. A need exists for porous metaland inorganic articles that have high pore density in thick sections andare made using environmentally benign materials. A need exists forcontinuous porous metal and polycrystalline ceramic articles ofindefinite length. Further, a need exists for metal and ceramic meshes,tubes and sheets having hollow walls. Other needs satisfied by theinvention are improved methods of making metal foils, the foils therebyproduced, and methods of making an inorganic coating on an inorganicsubstrate.

SUMMARY OF THE INVENTION

This invention comprises processes for producing metal, ceramic andcermet articles from low viscosity suspensions and the articles producedthereby. In a first embodiment, the invention is a method for making ahollow article comprising the steps:

-   -   a) selecting a polymeric substrate from the group consisting of        a filament, a plurality of essentially parallel filaments, a        woven fabric sheet, a knitted fabric sheet, a non-woven fabric        sheet, a mesh, a film, a sheet, and an open cell foam, said        polymeric substrate having exterior surfaces and optionally        having interior surfaces;    -   b) coating said exterior and interior surfaces, if any, of said        substrate with an aqueous slurry comprising a gel forming binder        and inorganic particles, said inorganic particles being at least        one member of the group consisting of a ceramic powder, a metal        powder, a cermet powder, carbon nanontubes, metal whiskers,        ceramic whiskers, or their mixture to form a coated article,        said slurry being applied at a temperature above its gelation        temperature and having a viscosity at most 0.4 Pa-sec at the        application temperature;    -   c) optionally, removing excess slurry from the coated article;    -   d) cooling said slurry to a temperature below its gelation        temperature;    -   e) substantially drying the coated article;    -   f) optionally repeating steps b) through e) one or more times;    -   g) heating the coated article for periods of times, at        temperatures and in atmospheres sufficient to vaporize the        polymeric substrate forming a green hollow article;    -   h) sintering the green hollow article in a protective atmosphere        under conditions of time and temperatures as are required to        cohere the inorganic particles into a unitary hollow article        having interior surfaces with essentially the initial form of        the vaporized substrate.

In a second embodiment, the invention is a hollow article produced bythe above method.

In a third embodiment, the invention is an open cell metal foam articlecomprising a multiplicity of hollow structural members, the externalsurfaces of said structural members defining connected pores, the numberof pores per centimeter in at least one direction satisfying thefollowing inequality:(55−8.27L)≦Pores/cm≦(140−8.27L)wherein L is the minimum dimension of said article selected from itslength, width, or thickness measured in centimeters.

In a fourth embodiment, the invention is an open cell inorganic hollowfoam article selected from the group consisting of a metal foam, aceramic foam and a cermet foam, said foam comprising a multiplicity ofhollow structural members, the external surfaces of said structuralmembers defining connected pores, the number of pores in at least onedirection satisfying the following inequalities:(55−1.3L)≦Pores/cm≦(140−1.3L)Pores/cm≧42wherein L is the minimum dimension of said article selected from itslength, width, or thickness, measured in centimeters.

In a fifth embodiment, the invention is an inorganic hollow articlecomprising a material selected from the group consisting of a metal, aceramic and a cermet or their combination, said article having a densityat least 50% of the density of the material of which said article iscomposed, said article having internal walls defining a network ofinterconnected channels, said channels permitting the passage of fluids,and said channel volume being from 0.1% to 30% of the total volume ofsaid article.

In a sixth embodiment, the invention is an inorganic hollow mesh articleselected from the group consisting of a metal mesh, a ceramic mesh and acermet mesh, said mesh comprising a multiplicity of hollow structuralmembers, the external surfaces of said structural members defining theboundaries of openings in said mesh.

In a seventh embodiment, the invention is an inorganic hollow sheetarticle selected from the group consisting of a hollow metal sheet, ahollow ceramic sheet, and a hollow cermet sheet, said article comprisingupper and lower integrally connected sheet members, said upper and lowersheet members defining the boundaries of at least one interior openvolume.

In an eighth embodiment, the invention is an inorganic hollow fiber ofcomprising at least one member of the group consisting of a metal, apolycrystalline ceramic, a cermet or their mixture, said fiber havingone or more inner surfaces separated by impervious walls from oneanother and from an outer surface, said inner surfaces defining one ormore continuous longitudinal channels along the length of the fiber,said walls being of isotropic structure, and said walls having uniformcomposition in the radial direction.

In a ninth embodiment, the invention is a method of making a metal foilcomprising the steps:

-   -   a) coating one surface of a substrate with an aqueous slurry        comprising a gel forming binder and a metal powder, said slurry        being applied at a temperature above its gelation temperature        and having a viscosity less than or equal to 0.4 Pa-sec at the        application temperature;    -   b) cooling said slurry to a temperature below its gelation        temperature to form a gel;    -   c) substantially drying the gel to form a green foil body;    -   d) optionally, separating the green foil body from the        substrate;    -   e) heating the green foil body for periods of times, at        temperatures, and in atmospheres sufficient to sinter the green        foil body into body into a dense metal foil, and optionally to        vaporize the substrate.

In a tenth embodiment, the invention is a method of making an inorganiccoating on an inorganic substrate comprising the steps:

-   -   a) coating at least one surface of said substrate with an        aqueous slurry comprising a gel forming binder and least one        member of the group consisting of a ceramic powder, a metal        powder, a cermet powder, carbon nanontubes, metal whiskers,        ceramic whiskers, or their mixture to form a coating on the        substrate, said slurry being applied at a temperature above its        gelation temperature and having a viscosity less than or equal        0.4 Pa-sec at the application temperature;    -   b) optionally, removing excess slurry from the substrate;    -   c) cooling said slurry to a temperature below its gelation        temperature;    -   d) substantially drying the coated substrate;    -   e) heating the coated substrate for periods of times, in        atmospheres, and at temperatures below the melting point of said        substrate sufficient to sinter the coating and the substrate        into a unitary article.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic diagram of a process for producing continuousarticles of the invention of indefinite length.

FIG. 2 is photomicrograph of a hollow fiber of the invention.

FIG. 3 is an illustration of a simple hollow sheet article of theinvention.

FIG. 4 a illustrates a plan view of a hollow sheet article where theinternal volume is divided into parallel channels.

FIG. 4 b illustrates a side view of the same hollow sheet article of theinvention where the internal volume is divided into parallel channels.

FIG. 5 is an illustration of a hollow sheet article where the upper andlower sheets are concentric tubes.

FIG. 6 is a photomicrograph of a hollow sheet article having twonon-intersecting sets of multiple internal channels running indirections generally perpendicular to each other.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises processes for producing metal, ceramic andcermet articles from low viscosity suspensions and the articles producedthereby. The articles of the invention possess novel structures.

As has been noted above, the penetration of porous media by pure liquidsis sensitively enhanced by lower viscosity of the liquid. However,another issue faced by the skilled man seeking to coat a foam with aslurry is that slurry stability and homogeneity is enhanced by higherviscosity of the liquid medium. Thus, a conflict exists between abilityto penetrate thick sections of a foam and the stability and homogeneityof slurries. This conflict is resolved in the present invention by useof gel-forming aqueous liquid media of very low viscosity that permithigh but yet uniform penetration of porous bodies.

In a first embodiment, the invention is a method for making a hollowarticle comprising the steps:

-   -   a) selecting a polymeric substrate from the group consisting of        a filament, a plurality of essentially parallel filaments, a        woven fabric sheet, a knitted fabric sheet, a non-woven fabric        sheet, a mesh, a film, a sheet, and an open cell foam, said        polymeric substrate having exterior surfaces and optionally        having interior surfaces;    -   b) coating said exterior and interior surfaces, if any, of said        substrate with an aqueous slurry comprising a gel forming binder        and inorganic particles, said inorganic particles being at least        one member of the group consisting of a ceramic powder, a metal        powder, a cermet powder, carbon nanontubes, metal whiskers,        ceramic whiskers, or their mixture to form a coated article,        said slurry being applied at a temperature above its gelation        temperature and having a viscosity less than or equal to 0.4        Pa-sec at the application temperature;    -   c) optionally, removing excess slurry from the coated article;    -   d) cooling said slurry to a temperature below its gelation        temperature;    -   e) substantially drying the coated article;    -   f) optionally repeating steps b) through e) one or more times;    -   g) heating the coated article for periods of times, at        temperatures and in atmospheres sufficient to vaporize the        polymeric substrate forming a green hollow article;    -   h) sintering the green hollow article in a protective atmosphere        under conditions of time and temperatures as are required to        cohere the inorganic particles into a unitary hollow article        having interior surfaces with essentially the initial form of        the vaporized substrate.

For the purposes of the invention, a gel is a substance consisting of atleast 25 vol. % liquid that can resist a small shearing force withoutflowing. A gel forming binder is a substance that can be dispersed in aliquid at a first temperature, and when this dispersion is cooled to asecond temperature termed the gelation temperature, the dispersionbecomes a gel.

The polymeric substrate may be of any length and the article of theinvention may be prepared in a batch mode or in a continuous mode.Preferably, the polymeric substrate is a continuous substrate ofindefinite length passing continuously through an apparatus in whichsteps b) through e) of the above embodiment are carried out to form acontinuous dried coated article of indefinite length, and thiscontinuous dried coated article is passed continuously through anapparatus in which steps g) through h) of the above embodiment arecarried out to form a continuous unitary hollow article of indefinitelength having interior surfaces with essentially the initial form of thevaporized substrate.

The continuous process is illustrated schematically in FIG. 1. Water 10,inorganic particles 20 and other constituents such as defoaming agentsand surfactants 30 are fed to an agitated vessel 100 in which a slurryis prepared having a viscosity of at most 0.4 Pa-s. The slurry 40 is fedto an immersion tank 110 to maintain a level in the tank above the levelof a pair of rolls 115. The slurry in the immersion tank is agitated tomaintain homogeneity. A polymeric substrate 300 is fed continuously intothe immersion tank 110 by means of the rolls 115. The polymericsubstrate 350 issuing from the immersion tank 110 has its exteriorsurface and interior surfaces coated by the slurry. Seals (not shown)prevent leakage of the slurry from the tank. The coated polymericsubstrate 350 is then passed into an oven 135 with heating element 137through an air stream 70, 72 to substantially dry water from thesubstrate. The dried-coated polymeric substrate 400 may be collected asa continuous article of indefinite length for further processing at alater time, or it may be passed immediately into a multi-compartmentoven 140 (illustrated) or a series of separate ovens (not illustrated).

The multi-compartment oven 140 provides separate temperature zones bymeans of individually controlled internal heaters 150 and eachcompartment is provided with its own gas 80, 85, 90 or vacuum 82, 87, 92connections so as to provide different chemical atmospheres or vacuum.Seals between compartments permit continuous passage of the coatedpolymeric substrate from compartment to compartment. The lastcompartment may be operated as a cool down zone.

The continuous dried coated polymeric substrate of indefinite length 400passes through the multi-compartment oven at a speed (residence time),and at temperatures and in atmospheres sufficient first to vaporize thepolymeric substrate leaving a green inorganic body, and then sufficientto sinter the green inorganic body into a continuous unitary article ofindefinite length 500 having impervious structural members.

The following comments apply to both a batch mode and a continuous modeof this embodiment.

The method of coating the polymeric substrate with the slurry ofinorganic particles may be by impregnation, immersion, painting,printing, or a combination of these or other methods.

After coating the polymeric substrate, excess slurry is optionallyremoved from the coated article by any practicable means. Excess slurrymay be removed from the substrate, for example, by blowing with a highvelocity gas stream, by squeezing, by squeegeeing, by applying asqueezing rolling pressure, by applying a vacuum, or by centrifugation.

Among the differences of the inventive process from the prior art is theuse of aqueous slurries that are gel forming and of very low viscosity.These differences permit the production of green articles that arerobust and flexible and sintered articles with generally thinnerstructural members and/or with larger transverse dimensions. Otheradvantages include simplicity and ease of production and use ofenvironmentally benign materials. Still other advantages will becomeevident.

Preferably, the aqueous slurry of inorganic particles has a viscosityless than 0.1 Pa-sec and most preferable less than 0.04 Pa-sec at theapplication temperature. The viscosity of the aqueous slurry dependsupon the solids content of the slurry, the shape and density of theinorganic particles, the type and concentration of the gel-formingbinder and the temperature of the slurry. The skilled man can readilyadjust these parameters to obtain a slurry viscosity less than or equalto 0.4 Pa-s. For example, Table I below shows the relationship of slurryviscosity to solids content for aqueous slurries of type 316 stainlesssteel powder (Grade PF-3F from Atmix Corp. and distributed by U.S.Bronze Co., Flemington, N.J.) having an average particle size of 3microns. The slurries in Table I were prepared by dissolving gelatin inwater at 30° C., then adding the stainless steel powder with agitationuntil the slurry consisted of 85.5 wt. % stainless steel powder, 13.5wt. % water, and 1 wt. % gelatin (Type A Gelatin supplied by GelitaNorth America, Sergeant Bluff, Iowa). Additional water was then added tothe slurry in steps to bring the solids content down with the slurryviscosity measured at each step. Slurry viscosities were measured at 25°C. Gelation temperature of the slurry was 22° C.

TABLE I Viscosity of 316 Stainless Steel Aqueous Slurries Wt. % 316 SSSlurry Viscosity, Powder Wt. % gelatin Pa · -s 85.5 1.0 0.4 80 0.9360.09 75 0.877 0.025 68 0.795 0.01

Preferably, the application temperature of the coating is less than 80°C., more preferably less than 60° C., and most preferably less than 40°C. The application temperature is above the gelation temperature of theslurry. Preferably, the gelation temperature of the slurry is aboveabout 20° C.

The gel forming binder is preferably at least one selected from thegroup consisting of agaroids, proteins, starches, polysaccharides,methyl cellulose, polyvinyl alcohol, polyacrylamide, and mixturesthereof. More preferably, the gel forming binder is gelatin orKappa-carrageenan.

Preferably, the concentration of the gel forming binder in the aqueousslurry is from 0.3% to 10% of the total weight of the slurry.

Preferably, the inorganic particles are selected from the groupconsisting of carbides, oxides, nitrides, borides and silicides ofmetals, nonmetals, and mixtures thereof.

Alternatively, it is preferred that the inorganic particles are selectedfrom the group consisting of pure metals, ferrous and non-ferrousalloys, intermetallic compounds and mixtures thereof. More preferableare metal particles selected from the group consisting of chromium,cobalt, copper, gold, iron, lead, manganese, molybdenum, nickel,niobium, palladium, platinum, rhodium, silver, tin, titanium, tungsten,vanadium, zinc and their alloys. Most preferred are metal particlesselected from the group consisting of type 316 stainless steel, alloy17-4PH, titanium carbide, tungsten carbide, tungsten-copper alloys andmolybdenum-copper alloys.

Preferably, the aqueous slurry additionally comprises about 0.05 toabout 1.2 percent by weight of water of a corrosion inhibitor selectedfrom the group consisting of sodium chromate, magnesium chromate,potassium chromate, sodium silicate, potassium silicate, potassiumnitrite and sodium nitrite. Additionally, the aqueous slurry may includebiocides, defoaming agents, wetting agents, thickening agents and othermaterials commonly incorporated in slurries or suspensions.

Excess slurry is optionally removed from the coated polymeric substrate.This may be accomplished, for example, by blowing with a high velocityair stream, by squeegeeing, by applying a rolling pressure or passingthrough nip rolls. FIG. 1 illustrates the option of passing the coatedpolymeric substrate under a high velocity air knife 120 fed by an airstream 60 to remove excess slurry. A catch basin (not shown) collectsthe excess slurry.

In a second embodiment, the invention is a hollow article produced bythe above method in either a continuous or batch mode. When thepolymeric substrate is a filament, the article formed by the aboveprocess is a thin wall hollow fiber. When the polymeric substrate is aplurality of essentially parallel filaments, the article formed is ahollow body having a plurality of continuous channels. When thepolymeric substrate is a woven, knitted or non-woven fabric sheet or amesh, the article formed may be a mesh having thin wall hollowstructural members or a hollow sheet depending on whether the openingsof the fabric are filled by the slurry. When the polymeric substrate isa film or a sheet, the article formed is a hollow film or sheet havingthin walls. When the polymeric substrate is an open cell foam, thearticle formed is a porous material having hollow thin wall structuralmembers. Preferably, the hollow article is one selected from the groupconsisting of a foam, a fiber, a mesh and a sheet.

In a third embodiment, the invention is an open cell metal foam articlecomprising a multiplicity of hollow structural members, the externalsurfaces of said structural members defining connected pores, the numberof pores per centimeter in at least one direction satisfying thefollowing inequality:(55−8.27L)≦Pores/cm≦(140−8.27L)  Eq. 1wherein L is the minimum dimension of said article selected from itslength, width, or thickness measured in centimeters. Preferably, thenumber of pores per centimeter in at least one direction satisfies thefollowing inequality:(60−8.27L)≦Pores/cm≦(120−8.27L)  Eq. 2

Preferably, the metal comprising the open cell metal foam article isselected from the group consisting of pure metals, ferrous andnon-ferrous alloys, intermetallic compounds and mixtures thereof. Morepreferably, the metal comprising the article is selected from the groupconsisting of chromium, cobalt, copper, gold, iron, lead, manganese,molybdenum, nickel, niobium, palladium, platinum, rhodium, silver, tin,titanium, tungsten, vanadium, zinc and their alloys. Most preferably,the metal comprising the articles is selected from the group consistingof type 316 stainless steel, alloy 17-4PH, titanium carbide, tungstencarbide, tungsten-copper alloys and molybdenum-copper alloys.

In a fourth embodiment, the invention is an open cell inorganic hollowfoam article selected from the group consisting of a metal foam, aceramic foam and a cermet foam, said foam comprising a multiplicity ofhollow structural members, the external surfaces of said structuralmembers defining connected pores, the number of pores in at least onedirection satisfying the following inequalities:(55−1.3L)≦Pores/cm≦(140−1.3L)  Eq. 3aPores/cm≧42  Eq. 3bwherein L is the minimum dimension of said article selected from itslength, width, or thickness, measured in centimeters.

It is preferred that a metal comprising the inorganic hollow foam ofthis embodiment is selected from the group consisting of pure metals,ferrous and non-ferrous alloys, intermetallic compounds and mixturesthereof. More preferable is a metal selected from the group consistingof chromium, cobalt, copper, gold, iron, lead, manganese, molybdenum,nickel, niobium, palladium, platinum, rhodium, silver, tin, titanium,tungsten, vanadium, zinc and their alloys. Most preferred is a metalselected from the group consisting of type 316 stainless steel, alloy17-4PH, titanium carbide, tungsten carbide, tungsten-copper alloys andmolybdenum-copper alloys.

Preferably, a ceramic comprising the inorganic hollow foam of thisembodiment is selected from the group consisting of carbides, oxides,nitrides, borides and silicides of metals, nonmetals, and mixturesthereof.

In a fifth embodiment, the invention is an inorganic hollow articlecomprising a material selected from the group consisting of a metal, aceramic and a cermet or their combination, said article having a densityat least 50% of the density of the material of which said article iscomposed, said article having internal walls defining a network ofinterconnected channels, said channels permitting the passage of fluids,and said channel volume being from 0.1% to 30% of the total volume ofsaid article.

Preferably, the article has a density at least 75% of the density of thematerial of which the article is composed and the channel volume is from0.1% to 10% of the total volume of the article.

It is preferred that a metal comprising the inorganic porous article ofthis embodiment is selected from the group consisting of pure metals,ferrous and non-ferrous alloys, intermetallic compounds and mixturesthereof. More preferable is a metal selected from the group consistingof chromium, cobalt, copper, gold, iron, lead, manganese, molybdenum,nickel, niobium, palladium, platinum, rhodium, silver, tin, titanium,tungsten, vanadium, zinc and their alloys. Most preferred is a metalselected from the group consisting of type 316 stainless steel, alloy17-4PH, titanium carbide, tungsten carbide, tungsten-copper alloys andmolybdenum-copper alloys.

Preferably, a ceramic comprising the inorganic porous article of thisembodiment is selected from the group consisting of carbides, oxides,nitrides, borides and silicides of metals, nonmetals, and mixturesthereof.

In a sixth embodiment, the invention is an inorganic hollow mesh articleselected from the group consisting of a metal mesh, a ceramic mesh and acermet mesh, said mesh comprising a multiplicity of hollow structuralmembers, the external surfaces of said structural members defining theboundaries of openings in said mesh.

It is preferred that a metal comprising the inorganic hollow mesh ofthis embodiment is selected from the group consisting of pure metals,ferrous and non-ferrous alloys, intermetallic compounds and mixturesthereof. More preferable is a metal selected from the group consistingof chromium, cobalt, copper, gold, iron, lead, manganese, molybdenum,nickel, niobium, palladium, platinum, rhodium, silver, tin, titanium,tungsten, vanadium, zinc and their alloys.

Most preferred is a metal selected from the group consisting of type 316stainless steel, alloy 17-4PH, titanium carbide, tungsten carbide,tungsten-copper alloys and molybdenum-copper alloys.

Preferably, the ceramic comprising the inorganic hollow mesh of thisembodiment is selected from the group consisting of carbides, oxides,nitrides, borides and silicides of metals, nonmetals, and mixturesthereof.

In a seventh embodiment, the invention is an inorganic hollow sheetarticle selected from the group consisting of a hollow metal sheet, ahollow ceramic sheet, and a hollow cermet sheet, said article comprisingupper and lower integrally connected sheet members, said upper and lowersheet members defining the boundaries of at least one interior openvolume. The upper and lower sheet members of this article may beconcentric tubes. The inorganic hollow sheet article may optionally bebonded to a substrate.

FIGS. 3, 4 and 5 illustrate three embodiments of the hollow sheetarticles of the invention. The hollow sheet articles of the inventionare useful as cooling jackets, heat pipes and for other applications.

It is preferred that a metal comprising the inorganic hollow sheet ofthis embodiment is selected from the group consisting of pure metals,ferrous and non-ferrous alloys, intermetallic compounds and mixturesthereof. More preferable is a metal selected from the group consistingof chromium, cobalt, copper, gold, iron, lead, manganese, molybdenum,nickel, niobium, palladium, platinum, rhodium, silver, tin, titanium,tungsten, vanadium, zinc and their alloys. Most preferred is a metalselected from the group consisting of type 316 stainless steel, alloy17-4PH, titanium carbide, tungsten carbide, tungsten-copper alloys andmolybdenum-copper alloys.

Preferably, a ceramic comprising the inorganic hollow sheet of thisembodiment is selected from the group consisting of carbides, oxides,nitrides, borides and silicides of metals, nonmetals, and mixturesthereof.

In an eighth embodiment, the invention is an inorganic hollow fiber ofcomprising at least one member of the group consisting of a metal, apolycrystalline ceramic, a cermet or their mixture, said fiber havingone or more inner surfaces separated by impervious walls from oneanother and from an outer surface, said inner surfaces defining one ormore continuous longitudinal channels along the length of the fiber,said walls being of isotropic structure, and said walls having uniformcomposition in the radial direction.

Preferably, the inorganic hollow fiber of this embodiment is a thin wallhollow fiber having walls with a thickness of from 2 to 100 microns,preferably from 3 to 30 microns.

Preferably, the inorganic hollow fiber of this embodiment is acontinuous hollow fiber of indefinite length.

It is preferred that a metal comprising the continuous inorganic hollowfiber of this embodiment is selected from the group consisting of puremetals, ferrous and non-ferrous alloys, intermetallic compounds andmixtures thereof. More preferable is a metal selected from the groupconsisting of chromium, cobalt, copper, gold, iron, lead, manganese,molybdenum, nickel, niobium, palladium, platinum, rhodium, silver, tin,titanium, tungsten, vanadium, zinc and their alloys. Most preferred is ametal selected from the group consisting of type 316 stainless steel,alloy 17-4PH, titanium carbide, tungsten carbide, tungsten-copper alloysand molybdenum-copper alloys.

Preferably, a polycrystalline ceramic comprising the continuousinorganic hollow fiber of this embodiment is selected from the groupconsisting of carbides, oxides, nitrides, borides and silicides ofmetals, nonmetals, and mixtures thereof.

In a ninth embodiment, the invention is a method of making a metal foilcomprising the steps:

-   -   a) coating one surface of a substrate with an aqueous slurry        comprising a gel forming binder and a metal powder, said slurry        being applied at a temperature above its gelation temperature        and having a viscosity less than or equal to 0.4 Pa-sec at the        application temperature;    -   b) cooling said slurry to a temperature below its gelation        temperature to form a gel;    -   c) substantially drying the gel to form a green foil body;    -   d) optionally, separating the green foil body from the        substrate;    -   e) heating the green foil body for periods of times, at        temperatures, and in atmospheres sufficient to sinter the green        foil body into body into a dense metal foil, and optionally to        vaporize the substrate.

It is preferred that the metal comprising the foil of this embodiment isselected from the group consisting of pure metals, ferrous andnon-ferrous alloys, intermetallic compounds and mixtures thereof. Morepreferable is a metal selected from the group consisting of chromium,cobalt, copper, gold, iron, lead, manganese, molybdenum, nickel,niobium, palladium, platinum, rhodium, silver, tin, titanium, tungsten,vanadium, zinc and their alloys. Most preferred is a metal selected fromthe group consisting of type 316 stainless steel, alloy 17-4PH, titaniumcarbide, tungsten carbide, tungsten-copper alloys and molybdenum-copperalloys.

In a tenth embodiment, the invention is a method of making an inorganiccoating on an inorganic substrate comprising the steps:

-   -   a) coating at least one surface of said substrate with an        aqueous slurry comprising a gel forming binder and least one        member of the group consisting of a ceramic powder, a metal        powder, a cermet powder, carbon nanontubes, metal whiskers,        ceramic whiskers, or their mixture to form a coating on the        substrate, said slurry being applied at a temperature above its        gelation temperature and having a viscosity less than or equal        to 0.4 Pa-sec at the application temperature;    -   b) optionally, removing excess slurry from the substrate;    -   c) cooling said slurry to a temperature below its gelation        temperature;    -   d) substantially drying the coated substrate;    -   e) heating the coated substrate for periods of times, in        atmospheres, and at temperatures below the melting point of said        substrate sufficient to sinter the coating and the substrate        into a unitary article.

It is preferred that a metal comprising the inorganic coating of thisembodiment is selected from the group consisting of pure metals, ferrousand non-ferrous alloys, intermetallic compounds and mixtures thereof.More preferable is a metal selected from the group consisting ofchromium, cobalt, copper, gold, iron, lead, manganese, molybdenum,nickel, niobium, palladium, platinum, rhodium, silver, tin, titanium,tungsten, vanadium, zinc and their alloys. Most preferred is a metalselected from the group consisting of type 316 stainless steel, alloy17-4PH, titanium carbide, tungsten carbide, tungsten-copper alloys andmolybdenum-copper alloys.

Preferably, a ceramic comprising the inorganic coating of thisembodiment is selected from the group consisting of carbides, oxides,nitrides, borides and silicides of metals, nonmetals, and mixturesthereof.

The following examples are presented to provide a more completeunderstanding of the invention. The specific techniques, conditions,materials, proportions and reported data set forth to illustrate theprinciples of the invention are exemplary and should not be construed aslimiting the scope of the invention.

EXAMPLES Example 1

316 Stainless Steel Hollow Foam

An open cell polyurethane foam block (Grade: Ultrafine) was obtainedfrom E.N. Murray Co., Denver, Colo. having a pore density of 43.3pores/cm. The interior and exterior surfaces of the foam block werecoated with an aqueous slurry of 316 stainless steel powder in thefollowing manner:

A 3.85 wt. % solution of gelatin (Type A Gelatin supplied by GelitaNorth America, Sergeant Bluff, Iowa) was prepared at 30° C. in watercontaining 0.0032 wt. % of potassium sorbate as a biocide. The gelationtemperature of this solution was 23° C. Stainless steel powder, type 316having an average particle size of 3 microns (Grade PF-3F, Epson AtmixCorporation, Aomori-ken, Japan and distributed by U.S. Bronze Co.,Flemington, N.J.) was added to this solution along with a small amountof a silicone defoaming agent (Grade C-2290 New London Distributors,Inverness, Ill.) with vigorous stirring to produce a homogeneous slurry.The composition of the slurry was 76.218 wt. % 316 stainless steelpowder, 0.915 wt. % gelatin, 0.0015 wt. % potassium sorbate, 0.0004 wt.% defoaming agent and 22.8651 wt. % water. The viscosity of the slurrywas 0.03 Pa-s at 25° C. The foam block was immersed in the coatingslurry for several seconds coating the outside and inside surfaces.Excess slurry was removed by squeezing and then by blowing with air.During removal of the excess slurry, cooling of the foam block to roomtemperature (˜21° C.) caused the slurry to gel. The coated foam blockwas air-dried for several hours at room temperature.

The dried coated foam block was then transferred to an oven andsubjected to the time and temperature schedule in Table II. The times,and temperatures in the ambient atmospheres were sufficient to vaporizethe polyurethane substrate to form a green hollow article, and thensufficient to sinter the stainless steel powder into a unitary article.An article of the invention was formed having dimensions of 5.1 cm×5.1cm×2.54 cm, a pore density of 43.3 pores/cm and dense, thin wall, hollow316 stainless steel structural members with interior surfaces havingessentially the initial form of the vaporized substrate. The carboncontent of the stainless steel was less than 12 parts per million.

The article of the invention satisfied the inequality of Equation 1 asfollows:(55−8.27L)≦Pores/cm≦(140−8.27L)(55−8.27×2.54)≦43.3≦(140−8.27×2.54)33≦43.3≦119  Eq. 1

TABLE II Time and Temperature Schedule for Vaporization of the PolymerSubstrate and Sintering of the Stainless Steel Powder Heating orTemperature, Cooling Rate, ° C. ° C./hr Time, hrs Atmosphere  25-500 2002.38 nitrogen  500 hold 1.0 hydrogen 500-850 200 1.75 hydrogen  850 hold0.5 hydrogen  850-1330 150 3.2 hydrogen 1330 hold 1.0 hydrogen 1330-1000200 1.15 hydrogen 1000 hold 0.03 hydrogen 1000-25  200 4.88 hydrogen

Example 2

17-4 PH Stainless Steel Hollow Foam

Example 1 was repeated using the same polyurethane foam and the sameslurry composition except that the metal powder was 17-4 PH stainlesssteel having an average particle size of 8 microns (Epson AtmixCorporation, Aomori-ken, Japan). An article of the invention was formedhaving dimensions of 5.1 cm×5.1 cm×2.54 cm, a pore density of 43.3pores/cm and dense, thin wall, hollow 17-4 PH stainless steel structuralmembers with interior surfaces having essentially the initial form ofthe vaporized substrate. The article of the invention satisfied theinequality of Equation 1.

Example 3

Nickel Hollow Foam

Example 1 was repeated using the same polyurethane foam and the sameslurry composition except that the metal powder was nickel powder havingan average particle size of 3 microns (type 210H, from Novamet, Wyckoff,N.J.). The same process was followed to coat, to remove excess slurryand to dry the coated foam. The dried coated foam block was thentransferred to an oven and subjected to the time and temperatureschedule in Table III. The times, and temperatures in the ambientatmospheres were sufficient to vaporize the polyurethane substrate toform a green hollow article, and then sufficient to sinter the nickelpowder into a unitary article. An article of the invention was formedhaving dimensions of 5.1 cm×5.1 cm×2.54 cm, a pore density of 43.3pores/cm and dense, thin wall, hollow nickel structural members withinterior surfaces having essentially the initial form of the vaporizedsubstrate. The article of the invention satisfied the inequality ofEquation 1.

TABLE III Time and Temperature Schedule for Vaporization of the PolymerSubstrate and Sintering of the Nickel Powder Heating or Temperature,Cooling Rate, ° C. ° C./hr Time, hrs Atmosphere  25-500 200 2.38nitrogen  500 hold 2.0 hydrogen 500-900 200 1.80 hydrogen  900 hold 0.5hydrogen  900-1300 150 2.67 hydrogen 1300 hold 1.0 hydrogen 1300-1000200 1.5 hydrogen 1000 hold 0.03 hydrogen 1000-25  200 4.88 hydrogen

Example 4

Thick 316 Stainless Steel Hollow Foam

Example 1 is repeated using a block of the same polyurethane foam buthaving twice the thickness of Example 1. An article of the invention isformed having dimensions of 10.3 cm×10.3 cm×10.3 cm, a pore density of43.3 pores/cm and hollow 316 stainless steel structural members withinterior surfaces having essentially the initial form of the vaporizedsubstrate. The article of the invention satisfies the inequality ofEquation 1 as follows:(55−8.27L)≦Pores/cm≦(140−8.27L)(55−8.27×10.3)≦43.3≦(140−8.27×10.3)−30.2≦43.3≦54.8.  Eq. 1

Example 5

High Pore Density 316 Stainless Steel Hollow Foam

An open cell melamine-formaldehyde foam block, grade PMF-100, wasobtained from Polymer Technologies Inc., Newark, Del. having a poredensity of 86.6 pores/cm. The interior and exterior surfaces of the foamblock were coated with the same aqueous slurry of 316 stainless steelpowder as described in Example 1 by immersion in the slurry for severalminutes. Excess slurry was removed by centrifugation and the coated foamblock was dried and subjected to the same time and temperature scheduleas described in Example 1.

An article of the invention was formed having dimensions of 5.1 cm×5.1cm×2.54 cm, a pore density of 86.6 pores/cm and thin wall hollow 316stainless steel structural members with interior surfaces havingessentially the initial form of the vaporized substrate. The article ofthe invention satisfied the inequality of Equation 1 as follows:(55−8.27L)≦Pores/cm≦(140−8.27L)(55−8.27×2.54)≦86.6≦(140−8.27×2.54)33≦86.6≦119  Eq. 1

Example 6

Continuous 316 Stainless Steel Hollow Foam

A slurry is continuously prepared in a mixing vessel 100 as illustratedschematically in FIG. 1. The composition of the slurry is 76.218 wt. %316 stainless steel powder, 0.915 wt. % gelatin, 0.0015 wt. % potassiumsorbate, 0.0004 wt. % defoaming agent and 22.8651 wt. % water. Theviscosity of the slurry is 0.03 Pa-s at 25° C. The gelation temperatureof the slurry is 23° C. The slurry 40 passes into an immersion tank 110under level control to maintain the level in the immersion tank. Acontinuous length of polyurethane foam 300 having a pore density of 43.3pores/cm is passed into the immersion tank 110 and is totally immersedin the slurry. The polyurethane foam 350 issuing from the immersion tankhas its exterior surface and interior surfaces coated by the slurry. Thecoated polyurethane foam passes under an air knife 120 fed by air stream60 that removes excess slurry and cools the coating to beneath the gelpoint temperature. The coated polyurethane foam is passed into an oven135 where it is dried by an air stream 70, 72 at a temperature of 95° C.The dried-coated polyurethane foam 400 is then collected as a continuousroll of indefinite length.

At a later time, the above roll of dried-coated polyurethane foam 400 isfed into a multicompartment oven 140. The first oven compartment ismaintained at a temperature of 500° C. in a nitrogen flow 80, 82.Residence time of the coated polyurethane foam in this oven compartmentis one hour during which time the polyurethane foam is vaporized leavinga porous green body with hollow structural members. The porous greenbody passes continuously through this oven compartment through a sealinto a second oven compartment maintained at a temperature of 1330° C.in a hydrogen flow 85, 87 of about ten liters/hr. Residence time of thegreen body in this compartment is one hour during which time the porousgreen body is sintered into a unitary article. The sintered article ispassed into a third oven compartment where it is cooled to 300° C. in anitrogen flow 90, 92. Issuing from the third oven compartment is acontinuous hollow article of indefinite length, 30 cm wide by 2.54 cmthick, having hollow, impervious structural members and a pore densityof 43.3 pores/cm; the interior spaces of the structural members havingthe shape of the vaporized polyurethane foam.

Example 7

High Pore Density Alumina Hollow Foam

Alumina (Al₂O₃) powder having an average particle size of about 11microns designated Alcan C-90 was obtained from Alcan Inc., Montreal,Quebec, Canada. 75.0 wt. % of the alumina powder was mixed with 24.5 wt.% water and 0.5 wt. % Darvan 821A dispersant from R.T. Banderbilt Co.Norwalk, Conn. and milled in a milling jar with a polyurethane linercontaining half inch cylindrical alumina milling media. The viscosity ofthis mixture was measured as a function of milling time as shown inTable IV below.

TABLE IV Milling Time (hr.) Viscosity (Pa-s) 6.25 0.0082 12 0.0075 150.079 18 0.0074 21 0.0075 24 0.008 24.5 0.0099 25 0.011 25.5 0.0153 260.0188

At the end of 26 hours of milling, the average particle size of thealumina powder was 1 micron. A homogeneous coating slurry was formedfrom this milled alumina slurry by mixing 98.7981 wt. % of the milledalumina slurry with 1.2 wt. % gelatin, 0.001 wt. % potassium sorbate,and 0.0004 wt. % defoaming agent. The viscosity of the slurry was 0.022Pa-s at 27° C. The gelation temperature of this slurry was 23° C.

A block of the open cell melamine-formaldahyde foam described in Example5 having a pore density of 86.6 pores/cm was immersed in this aluminaslurry for several minutes. The interior and exterior surfaces of thefoam block were thereby coated. Excess slurry was removed bycentrifugation and the coated foam block was dried and subjected to thetime and temperature schedule shown in Table V.

TABLE V Time and Temperature Schedule for Vaporization of the PolymerSubstrate and Sintering of the Alumina Powder Heating or Temperature,Cooling Rate, ° C. ° C./hr Time, hrs Atmosphere 25-500 200 2.38 air 500hold 1.0 air 500-900  200 2.0 air 900 hold 0.5 air 900-1550 150 4.33 air1550  hold 1.5 air 1550-25   200 7.62 air

The times, and temperatures in the ambient atmospheres were sufficientto vaporize the melamine-formaldehyde substrate to form a green hollowarticle, and then sufficient to sinter the alumina powder into a unitaryarticle. An article of the invention was formed having dimensions of 5.1cm×5.1 cm×2.54 cm, a pore density of 86.6 pores/cm and dense, thin wall,hollow alumina structural members with interior surfaces havingessentially the initial form of the vaporized substrate. The article ofthe invention satisfied the inequality of Equation 1 as follows:(55−8.27L)≦Pores/cm≦(140−8.27L)(55−8.27×2.54)≦86.6≦(140−8.27×2.54)33≦86.6≦119  Eq. 1

Example 8

Microchannels in Block of 316 Stainless Steel

Example 1 was repeated using a 1 cm×1 cm×1 cm block of the samepolyurethane foam and the same 316 stainless steel slurry composition.The foam was immersed in the slurry, squeezed and then immersed again.After the second immersion, the slurry that filled the pore cavities ofthe foam was not removed. The slurry-filled foam was dried at roomtemperature for 24 hours. The dried coated foam block was thentransferred to an oven and subjected to the time and temperatureschedule in Table VI. The times, and temperatures in the ambientatmospheres were sufficient to vaporize the polyurethane substrate toform a green hollow article, and then sufficient to sinter the stainlesssteel powder into a unitary article.

TABLE VI Time and Temperature Schedule for Vaporization of the PolymerSubstrate and Sintering of the Stainless Steel Powder Heating orTemperature, Cooling Rate, ° C. ° C./hr Time, hrs Atmosphere  25-500 2002.38 nitrogen  500 hold 1.0 hydrogen 500-900 200 2.0 hydrogen  900 hold0.5 hydrogen  900-1315 150 2.77 hydrogen 1315 hold 1.0 hydrogen1315-1000 200 1.15 hydrogen 1000 hold 0.03 hydrogen 1000-25  200 4.88hydrogen

A specimen article of the invention was formed having a density 50% ofthat of solid 316 stainless steel and having a 3-dimensional network ofinterconnected channels. When the specimen was placed in a shallow poolof water containing red dye, drops of the red colored water appearedwithin seconds on its top surface showing that the channels permittedthe passage of fluids. Measurements of the dry weight of the article andthe weight when filled with liquid showed the channel volume to be 9.2%of the total volume of the article. Electric discharge machining of asurface of the specimen and examination by electron microscopy showedthe interconnected network of channels; the entrance holes of thechannels having the same triangular shape as the precursor polyurethanefoam ligaments.

This embodiment of the invention is useful as a filter among otherapplications.

Example 9

Microchannels in Block of 316 Stainless Steel

An open cell melamine-formaldahyde foam block, grade PMF-100, wasobtained from Polymer Technologies Inc., Newark, Del. having dimensionsof 0.7 cm×1 cm×2 cm and a pore density of 86.6 pores/cm. The foam blockwas totally immersed for several minutes in the same aqueous slurry of316 stainless steel powder as described in Example 1. The slurry-filledfoam was dried at room temperature for 24 hours. The dried coated foamblock was then transferred to an oven and subjected to the time andtemperature schedule in Table II. The times, and temperatures in theambient atmospheres were sufficient to vaporize themelamine-formaldehyde substrate to form a green hollow article, and thensufficient to sinter the stainless steel powder into a unitary article.

The edges of the article were machined using an electric dischargemethod (EDM) exposing a network of interconnecting channels. Theresulting block had dimensions of 0.615 cm×0.8 cm×1.03 cm, a volume of0.5068 cc and weighed 3.695 g. The block density of 7.29 g/cc was 91.1%of the density of solid 316 stainless steel.

When the specimen was placed in a shallow pool of water containing reddye, drops of the red colored water appeared within seconds on its topsurface showing that the channels permitted the passage of fluids.Measurements of the dry weight of the article and the weight when filledwith liquid showed the channel volume to be 0.32% of the total volume ofthe article.

Example 10

316 and 17-4PH Stainless Steel Microtubes

Nylon 6 monofilaments with various cross-sections, e.g., trilobal,hexalobal, hexagonal and round, were spun and collected. Lengths of thefibers were coated with either the 316 stainless steel slurry describedin Example 1, or with the 17-4 PH slurry described in Example 2, bydrawing the filaments through a bath of the slurry. Excess slurry wasremoved from the fibers by blowing with air and the coated fibers weredried at 25° C. for two hours. The coated and dried fibers weretransferred to an oven and subjected to the time and temperatureschedule shown in Table I above. The times, and temperatures in theambient atmospheres were sufficient to vaporize the nylon fibers to forma green hollow article, and then sufficient to sinter the stainlesssteel powders into unitary thin wall hollow tubes with cross-sectionshaving the same shape as the precursor nylon fibers. The walls of thetubes were impervious, isotropic in structure, and of uniformcomposition in the radial direction.

Scanning electron microscopy measurements of a 316 stainless steel tubeof the invention with a round cross-section showed its dimensions to be70 microns in inside diameter, 100 microns in outside diameter with awall thickness of 15 microns (FIG. 2).

The microtubes of the invention are useful as body implant devices amongother applications.

Example 11

Continuous Multi-Channel Stainless Steel Fiber

A continuous nylon 6 yarn is spun consisting of twenty-four filaments ofindefinite length and round cross-sections. The yarn is continuouslycoated with the 316 stainless steel slurry described in Example 6 and isdried, heated and sintered by the process described in Example 6 and asillustrated schematically in FIG. 1.

The unitary sintered fiber Issuing from the process is a continuoushollow 316 stainless steel fiber of indefinite length having twenty-fourcontinuous longitudinal channels separated by impervious structuralwalls having an isotropic structure, the channels having thecross-sectional shape of the vaporized nylon filaments and the wallshaving uniform composition in the radial direction.

Example 12

Multi-Channel Alumina Fiber

A length of polyester yarn consisting of 3200 twenty-five microndiameter filaments was coated on all surfaces with an alumina slurryhaving the same composition as in Example 7. The coated yarn was airdried, placed in an oven and subjected to the oven temperatures andatmospheres shown in Table V.

The unitary sintered fiber Issuing from the oven was a hollow aluminafiber having an external diameter of 2.95 mm and having 3200longitudinal channels along its length. The channels were separated byimpervious structural walls of isotropic structure and uniform structurein the radial direction, and having the cross-sectional shape of thevaporized polyester filaments.

Example 13

316 Stainless Steel Foils

The same 316 stainless steel slurry as described in Example 1 was coatedonto substrates consisting of a nylon hydrophilic polymer and dried at25° C. for three hours. The slurry coatings were applied at differentthickness'. The dried coatings were separated from the substrates,placed in an oven and heated and sintered under the conditions shown inTable II. The thickness' of sintered stainless steel foils so producedwere from 25 to 250 microns. The foils had more than 95% of thetheoretical density of 316 stainless steel.

Example 14

316 Stainless Steel Coating on an Alumina Substrate

The same 316 stainless steel slurry as described in Example 1 was coatedonto a substrate consisting of 3 cm×3 cm×1 mm fully dense alumina plateand dried at 25° C. for three hours. The coated alumina substrate wasplaced in an oven and heated and sintered under the conditions shown inTable II. A unitary article was produced consisting of a 316 stainlesssteel coating on an alumina plate.

Example 15

Copper Foils

Example 13 was repeated using the same slurry composition except thatthe metal powder was copper having an average particle size of 5 microns(Grade MIM 3123, UltraFine Techology Inc., Woonsocket, R.I.). The slurrycoatings were applied at different thickness'. The dried coatings wereseparated from the hydrophilic polymer substrates, placed in an oven andheated and sintered under the conditions shown in Table VII. Thethickness' of sintered copper foils so produced were from 15 to 100microns. The foils had more than 95% of the density of copper.

TABLE VII Time and Temperature Schedule for Sintering the Copper PowderHeating or Temperature, Cooling Rate, ° C. ° C./hr Time, hrs Atmosphere 25-500 200 2.38 nitrogen  500 hold 1.0 hydrogen 500-850 200 1.75hydrogen  850 hold 0.5 hydrogen  850-1330 150 2.4 hydrogen 1330 hold 1.0hydrogen 1315-1000 200 1.575 hydrogen 1000 hold 0.03 hydrogen 1000-25 200 4.88 hydrogen

Example 16

Tungsten/10% Copper Foils

Example 13 was repeated using the same slurry composition except thatthe metal powder was a mixture consisting of 90 wt. % tungsten powderhaving an average particle size of 5 microns (grade: C10 from BuffaloTungsten Inc., Depew, N.Y.) and 10 wt. % of the same copper powder as inExample 15. The slurry coatings were applied at different thickness'.The dried coatings were separated from the hydrophilic polymersubstrates, placed in an oven and heated and sintered under theconditions shown in Table VII. The foils had more than 96.5% of thetheoretical density of tungsten/10% copper. The theoretical density ofthe mixture is defined as follows:ρ_(Thear) =x _(W)ρ_(w) +x _(Cu)ρ_(Cu)

where: ρ_(w), ρ_(Cu) are the densities of pure tungsten and pure copper

-   -   x_(w), x_(Cu) are the weight fractions of tungsten and copper.

TABLE VIII Time and Temperature Schedule for Sintering the Tungsten/10%Copper Powder Heating or Temperature, Cooling Rate, ° C. ° C./hr Time,hrs Atmosphere  25-500 200 2.38 nitrogen  500 hold 1.0 hydrogen 500-900200 2.0 hydrogen  900 hold 0.5 hydrogen  900-1315 150 2.77 hydrogen 1315hold 1.0 hydrogen 1315-1000 200 1.58 hydrogen 1000 hold 0.03 hydrogen1000-25  200 4.88 hydrogen

Example 17

Tungsten/20% Cooper Foils

Example 15 was repeated using the same slurry composition except thatthe metal powder was a mixture consisting of 80 wt. % tungsten powderand 20 wt. % copper powder using the same tungsten and copper powders asin Example 15. The slurry coatings were applied at different thickness'.The dried coatings were separated from the hydrophilic polymersubstrates, placed in an oven and heated and sintered under the sameconditions as shown in Table VII. The foils had more than 97% of thetheoretical density of tungsten/20% copper.

Example 18

Simple Hollow Sheet

A nylon-6 sheet substrate having dimensions of 10 cm×10 cm×0.05 cm andseveral rows of 1 mm perforations along its length is coated byimmersion in the same 316 stainless steel slurry as described inExample 1. The perforations are also filled by the slurry. The sheet isair dried at 20° C. for several hours and then placed in an oven whereit is subjected to the same time and temperature schedule as shown inTable II. The times, and temperatures in the ambient atmospheres aresufficient to vaporize the nylon substrate to form a green hollowarticle, and then sufficient to sinter the stainless steel powder into aunitary article. As illustrated in FIG. 3, a hollow 316 stainless steelsheet article of the invention 300 is formed consisting of an uppersheet member 11 and a lower sheet member 12 integrally connected bystainless steel columns 13 where the perforations existed in the nylonsheet, the upper and lower sheet members defining the boundaries of anopen interior volume 14 having the shape of the vaporized substrate.

The hollow sheet is useful as a cooling jacket among other applications.

Example 19

Hollow Sheet Article having Internal Channels

Four nylon-6 sheet strips having dimensions of 10 cm×1 cm×0.05 cm areplaced parallel to each other in a shallow box of dimensions 10 cm×10cm×0.15 cm made of a hydrophilic material. The strips are placed atspacings of 1 cm from each other. The box is filled with the same 316stainless steel slurry as described in Example 1 completely surroundingthe nylon strips. The slurry is substantially dried at 20° C. and theresulting article is placed in an oven where it is subjected to the sametime and temperature schedule as shown in Table II. The times, andtemperatures in the ambient atmospheres are sufficient to vaporize thenylon strips to form a green hollow article, and then sufficient tosinter the stainless steel powder into a unitary sheet article. Asillustrated in FIGS. 4 a and 4 b, a hollow 316 stainless steel sheetarticle of the invention 400 is formed consisting of the upper 41 andlower members of the sheet 42 connected at two of their edges and bystainless steel spacers 43, the upper and lower sheet members definingan internal volume 44 having the shape of the vaporized nylon strips.

Example 20

Hollow Sheet Article having Sets of Perpendicular InterconnectedChannels

A plain weave nylon fabric having a thread count of 67 threads/cm inboth warp and woof directions was cut into a square having dimensions of10×10 cm. The fabric square was placed in a shallow box and covered withthe same 316 stainless steel slurry as described in Example 1 completelysurrounding the fabric square and filling the spaces between thethreads. The slurry was substantially dried at 20° C. and the resultingarticle was placed in an oven where it was subjected to the same timeand temperature schedule as shown in Table II. The times, andtemperatures in the ambient atmospheres were sufficient to vaporize thenylon fabric to form a green hollow article, and then sufficient tosinter the stainless steel powder into a unitary sheet article.

A photomicrograph of a cross-section of this article of the invention isshown in FIG. 6 along a plane that includes an axis of one of thefibers. It is seen that the article has two sets of interconnectedinternal channels that lie in generally perpendicular directions.

The article of the invention is useful as a heat exchanger among otherapplications.

Example 21

Hollow Sheet Article Comprising Concentric Tubes

A nylon-6 tube having an inside diameter of 9 mm, an outside diameter of10 mm and a length of 30 cm and several rows of 1 mm perforations alongits length is coated on its inside surface, and its outside surface byimmersion in the same 316 stainless steel slurry as described inExample 1. The perforations are also filled by the slurry. The coatedtube is air dried at for several hours at 25° C. and placed in an ovenwhere it is subjected to the same time and temperature schedule as shownin Table II. The times, and temperatures in the ambient atmospheres aresufficient to vaporize the nylon tube to form a green hollow article,and then sufficient to sinter the stainless steel powder into a unitaryarticle. As illustrated in FIG. 4, a hollow 316 stainless steel sheetarticle 700 of the invention is formed consisting of an upper sheetmember 31 and a lower sheet member 32 integrally connected by stainlesssteel columns 33 where the perforations existed in the nylon tube, theupper and lower sheet members defining the boundaries of an openinterior volume 34 having the shape of the vaporized nylon tube

Example 21

Open nylon-6 monofilament square mesh fabrics having various meshopenings and filament thickness' were obtained from Sefar America,Depew, N.Y. The mesh openings varied from 500 to 1000 microns and themonofilament diameters varied from 60 microns to 300 microns. The mesheswere dip coated with the 316 stainless steel slurry as described inExample 1. Excess slurry was removed from the meshes by blowing with agentle air flow to clear the openings between the monofilaments. Thecoated meshes were dried and sintered as described in Example 1 andTable II. Hollow stainless steel meshes were produced comprising amultiplicity of hollow structural members, the external surfaces of thestructural members defining the boundaries of openings in the meshes,and the internal surfaces of the structural members having the shape ofthe vaporized nylon meshes.

Having thus described the invention in rather full detail, it will beunderstood that such detail need not be strictly adhered to but thatfurther changes, modifications and uses may suggest themselves to oneskilled in the art, all falling within the scope of the invention asdefined by the subjoined claims.

1. A method for making a hollow foam article comprising the steps: a)selecting a polymeric substrate consisting of an open cell foam, saidpolymeric substrate having exterior surfaces and interior surfaces; b)coating said exterior and interior surfaces of said foam substrate withan aqueous slurry comprising a gel forming binder and inorganicparticles, said inorganic particles being at least one member of thegroup consisting of a ceramic powder, a metal powder, a cermet powder,carbon nanotubes, metal whiskers, ceramic whiskers, or their mixture toform a coated article, said slurry being applied at a temperature aboveits gelation temperature and at a temperature less than about 60° C.,said slurry having a viscosity less than or equal to 0.1 Pa-sec at theapplication temperature; c) optionally, removing excess slurry from thecoated foam article; d) cooling said slurry to a temperature below itsgelation temperature; e) substantially drying the coated foam article;f) optionally repeating steps b) through e) one or more times; g)heating the coated article for periods of times, at temperatures and inatmospheres sufficient to vaporize the polymeric substrate forming agreen hollow foam article; h) sintering the green hollow foam article ina protective atmosphere under conditions of time and temperatures as arerequired to cohere the inorganic particles into a unitary hollow foamarticle having hollow structural members with interior surfaces havingessentially the initial form of the vaporized substrate.
 2. The methodof claim 1, wherein said polymeric foam substrate is a continuoussubstrate of indefinite length passing continuously through an apparatusin which steps b) through f) of claim 1 are carried out to form acontinuous dried coated foam article of indefinite length; and whereinthis continuous dried coated foam article is afterward passedcontinuously through an apparatus in which steps g) through h) of claim1 are carried out to form a continuous unitary hollow foam article ofindefinite length having hollow structural members with interiorsurfaces having essentially the initial form of the vaporized substrate.3. The method of claim 1 wherein said unitary hollow foam article is anopen cell metal foam article comprising a multiplicity of hollowstructural members, the external surfaces of said structural membersdefining connected pores, the number of pores per centimeter in at leastone direction satisfying the following inequality:(55−8.27L)≦Pores/cm≦(140−8.27L) wherein L is the minimum dimension ofsaid article selected from its length, width, or thickness measured incentimeters.
 4. The method of claim 1 wherein said unitary hollow foamarticle is an open cell inorganic hollow foam article selected from thegroup consisting of a metal foam, a ceramic foam and a cermet foam, saidfoam comprising a multiplicity of hollow structural members, theexternal surfaces of said structural members defining connected pores,the number of pores in at least one direction satisfying the followinginequalities:(55−1.3L)≦Pores/cm≦(140−1.3L) and Pores/cm≧42 wherein L is the minimumdimension of said article selected from its length, width, or thickness,measured in centimeters.
 5. The method of claim 1 wherein excess slurryis not removed from the coated foam article, said excess slurrysubstantially filling the pore volume; and wherein said unitary hollowfoam article has internal walls defining a network of interconnectedchannels, said channels permitting the passage of fluids, said articlehaving a density at least 50% of the density of the material of whichsaid unitary hollow foam article is composed, and said channel volumebeing from 0.1% to 30% of the total volume of said unitary hollow foamarticle.